Polyurethane compositions obtained by curing liquid isocyanate-terminated prepolymers are finding increased use in diverse applications requiring castable elastomers. Because of their inherent toughness, outstanding resistance to abrasion, oils, solvents, chemicals, oxidation, and the wide range of hardness and flexibility from soft, flexible elastomers to rigid plastic, they are frequently used in numerous applications such as: abrasion resistant coatings; coatings on metal or fabric for belting; flexible mechanical couplings, gears and drive wheels; mallet and hammer heads; rollers for printing and feed conveying; shock absorbent pads and bumpers; solid industrial truck tires and caster wheels; and the like.
The polyurethane elastomers are linear multiblock copolymers of the (AB).sub.x type structure. They are typically formed as the reaction product of a diisocyanate with a hydroxyl terminated polyether or polyester polyol and a low molecular weight chain extender. The elastomeric properties of these materials arise from phase separation which leads to the formation of hard and soft segments. The soft segment consist of a long polyether or polyester chain with a molecular weight in the range of 1000-2000 while the hard segments are derived from the diisocyanate (MDI, i.e. 4,4'-diphenylmethane diisocyanate or TDI, i.e. tolylene diisocyanate) and simple glycols, triols, or diamines as curing agents.
The most commonly used amine curing agents for the castable elastomers are hindered or electronegatively substituted aromatic diamines of which 3,3'-dichlorobenzidine, 4,4'-methylenebis(2-chloroaniline) and 2,5,3'-trichloro-4,4'-diaminodiphenylmethane are representative. To a lesser extent, unhindered aromatic diamines such as 4,4'-methylenedianiline and phenylenediamines are used. Some diamine curing agents possess long "pot life" which in certain cases facilitates molding operations.
There has been a need in the art, therefore, for novel chain extenders and curing agents having controlled reactivities under normal operating conditions to permit the desired degree of chain length and cure to be achieved within practical and economically attractive time ranges and having sufficiently low melting points to allow them to be readily blended with the isocyanate terminated prepolymers.